1. Field of the Invention
The present invention relates to a fiber laser processing method and a fiber laser processing apparatus for applying a pulsed fiber laser beam generated with the use of fiber laser to an object to be processed to perform desired laser processing.
2. Description of the Related Art
Recently, a laser processing method utilizing fiber laser attracts attention. Since the fiber laser uses a very elongated core provided in an optical fiber as an active medium, a laser beam (fiber laser beam) with a narrower beam diameter and a smaller beam divergence angle may be oscillated and output. Since excitation light made entering into a fiber passes over the core a number of times and exhausts the excitation energy while that light is propagated through a long optical path, the fiber laser beam may be generated with very high oscillation efficiency. Since the fiber laser generates no thermal lens effect in the core of the fiber, a beam mode of the fiber laser beam is very stable.
In general, for fiber laser used in a fiber laser processing apparatus, an oscillation optical fiber having a core doped with a rare-earth element is optically disposed between a pair of optical resonant mirrors; the core of the optical fiber is optically excited to reciprocate an oscillation light beam with a predetermined wavelength output from the end face of the core in the axial direction between the optical resonant mirrors a number of times for resonance and amplification; and a coherent laser beam is picked up from one of the optical resonant mirrors (partial reflection mirror or output mirror) to the outside. Normally, an optical lens is disposed between the fiber end face and the optical resonant mirror to converge (focus) the oscillation light beam reflected by the optical resonant mirror with the optical lens to return the light beam to the core end face of the oscillation optical fiber. To optically excite the core of the oscillation optical fiber, an LD end face excitation mode is employed by using a laser diode (LD) for an excitation light source and focusing and making the LD light (excitation light) incident on the core end face through the optical resonant mirror and the optical lens.
In the laser processing using a pulsed fiber laser beam, especially, the laser processing such as seam welding, cutting, and boring, a rising rate of an LD drive current is enhanced through performance improvement of an LD power source driving the excitation LD to increase a repetition frequency of the fiber laser beam and, for example, a repetition oscillation of about 5 kHz is enabled (see Japanese Patent Application Laid-Open Publication No. 2007-190566).
However, if an LD power source with the LD drive current having a higher rising rate is used for the laser processing using the pulsed fiber laser, a narrow pulse (high-peak pulse) HP with an abnormally high peak value may occur in the fiber laser beam at the rising edge although the pulsed LD drive current may be acquired with a wavelength in accordance with setting as shown in FIG. 5. The occurrence of such a high-peak pulse HP occurs may damage the core (core doped with a rare-earth element) of the oscillation optical fiber, which is an active medium of the fiber laser, and may also adversely affect processing performance/processing quality of applications using such a fiber laser beam.
To constrain the occurrence of the above high-peak pulse HP, a technique is considered to be effective that maintains the fiber laser in a low-power laser oscillation state by applying a constant base current to the LD power source while waiting the start of the laser processing after powering on a fiber laser processing apparatus. However, maintaining the fiber laser in a constant laser oscillation state has drawbacks of increasing power consumption and shortening the life of LD. Although the fiber laser beam has no risk of going out of the apparatus since a shutter blocks the optical path of the fiber laser beam in the apparatus during the waiting period, a risk to safety becomes problematic if the apparatus is handled by a user without knowing or inadvertently forgetting that the fiber laser is maintained in the laser oscillation state.